This invention is directed to a microwave heating device and in particular to a microwave applicator for sealing cardboard cartons.
Various industries use cardboard cartons for packaging their products (e.g. beer bottles, soup cans, etc.). The tops and bottoms of the boxes are commonly sealed by using molten hot melt. The hot melt for the bottom flaps is applied by a set of wheels dipping in a hot melt container kept at 350.degree. F. The hot melt for the top flaps is applied through a set of nozzles. The flaps are then pressed together to seal the box while the hot melt is still soft. This system has various drawbacks. The nozzles invariably get clogged and tie up the production lines. The hot melt has to be either maintained at 350.degree. F. at all times or long warm-up times are necessary after a weekend or an overnight shutdown.
To overcome these problems, it is proposed that the adhesive be applied during the carton manufacturing process. The carton may then be sealed by closing the flaps and heating the adhesive to reactivate the hot melt. This may presently be accomplished by applying a hot plate to the carton flaps, heating the adhesive and thereby sealing the carton when the adhesive cools. However, the use of hot plates in a packaging line would be impractical except for very thin cardboards.
It is therefore proposed that the adhesive be activated by applying microwave power to the carton where adhesive is present. U.S. Pat. No. 3,999,026 which issued on Dec. 21, 1976 to Boiling, teaches an applicator that consists of a resonator divided into at least two parallel chambers by a separating wall with the separating wall connected to a microwave energy source. The resonator further includes a slot in the resonator wall above the edge of the separating wall. This applicator produces even heating in a longitudinal heating area along the slot in the wall.